Many screw presses are known in the literature and in practice, especially in the food and chemical industry. Their common characteristic is that the revolving worm shaft carries the material to be separated in a housing with a perforated wall. The cross section of the housing narrows in the direction of the material transfer, thereby realizing the pressing effect. An impact element is used for increasing the pressing effect. The solid component of the material passing through the housing with a perforated wall leaves the apparatus at the impact element, while losing some of the material's liquid content. The liquid exits by way of the perforation.
Many alternative embodiments of screw presses are known. Each includes the above-noted basic elements, but fundamentally different solutions exist with respect to their details and supplementary units. The common disadvantage of the known apparatuses is the inefficient separation, as a result of which the required organoleptic and chemical purity cannot be ensured for the separated phases. The apparatuses are suitable mostly for one kind of separation process without being adaptable to other kinds. The processes (heat treatment, mixing of additives, etc.) preceding the pressing and determining the separation capability are carried out generally in several series-connected apparatuses, or in those built next to each other. Thus the processing time is prolonged, and the process becomes more expensive.
Such apparatuses are already known which are assembled with replaceable elements (for instance, according to the GFR patent specification No. 2,730,473), as well as such presses the screw of which is assembled from several parts (GFR patent specification No. 1,944,642). Such apparatuses also in which the housing is provided with a double jacket in order to adjust the temperature of the material within (GB patent patent specification No. 1251072).
These solutions are aimed at accomplishing as many different tasks as possible with the screw press and at providing a relatively wide range of variation of the pressing parameters.
However, the fundamental shortcoming of these solutions is the inefficient separation. This is due first of all to the constant size of the perforations of the elements. It is known that the perforated element acts in two different ways during its application: it retains the solid component and by compacting the solid component it prevents the liquid component from escaping. If the size of an element of solid material (the fiber) is substantially smaller than the size of the perforation, then a certain part of the solid material penetrates the perforation, sticking to the wall, while another part falls through the perforation, thereby reducing the efficiency of separation. At the same time the size of the perforation is gradually reduced by the adhering fibers. If the fibers are substantially larger than the perforation, they arch over the perforation and thereby increase the flow resistance. Naturally, many other variations may occur between the above two extreme cases, including the simultaneous occurrence of the extreme cases.
Formation of the suitable perforation is further aggravated by the fact that the layer of solid material in the immediate vicinity of the perforation has a texture that is fundamentally different from that of the rest of the solid material. This is due to the periodic development and destruction of the two extreme cases. The composition of this marginal layer is fundamentally determined by the size and distribution of the perforations. The filtering effect is inversely proportional to the joint resistance of the marginal layer of solid material and the perforated element.
Thus it becomes obvious that not only would each particular application of the screw press require a different perforation pattern, but in the optimal case even a single application would require variable size of the perforations.
Elements with variably sized perforations are also known. One such element is disclosed in the British patent specification No. 1,000,773. Here the perforated element consists of bars placed next to each other, between which the required gap size is maintained by pieces, e.g., balls. The fundamental disadvantage of this solution is that the gap size can be altered only by disassembly of the apparatus. This required procedure restricts the universal applicability of the apparatus, since changing the perforation size requires shutdown and significant downtime, and also it does not allow readjustment in response to changed conditions in the given technological process.